The present invention relates generally to the inspection of the surface of a workpiece for defects. More particularly, the present invention relates to an apparatus and method that scan the surface of a workpiece with an optical beam and process the optical signal reflected and detected from the surface of the workpiece prior to determining from the reflected optical signal whether the workpiece includes defects, thereby increasing the sensitivity of the inspection.
For most semiconductor microelectronic applications, an important factor is the feature size of devices located on a silicon integrated circuit die. These devices include transistors, such as MOSFET""s, metallization paths, as well as capacitors and resistors. In particular, for most applications, the smaller the size of the semiconductive device the better. Smaller device geometry enables higher transistor switching speeds due to smaller junction capacitance. It also enables the development of digital circuits with greater processing power and complexity. Just a few years ago, the typical feature size for a typical semiconductive transistor was approximately 300 nanometers (nm). However, more recently the feature size has been reduced to 180 nm, while some semiconductive transistors are now being manufactured with dimensions of 130 nm.
Although the miniaturization of semiconductor devices is typically advantageous, this reduction in size has imposed some problems in the manufacturing of these devices. Specifically, semiconductive devices are manufactured from crystalline silicon wafers that have been grown, sliced into wafers, and polished. Although the methods for creating these wafers and preparing them for semiconductor manufacture are quite advanced, contaminant particles may be introduced onto the surface of the wafer and/or contaminants or surface defects may be present in the wafer. These contaminants and/or defects, if large in dimension relative to the semiconductive device to be manufactured from the wafer, may result in device defects and reduced production yields. As such, as the size of semiconductive devices and their associated components are reduced, contaminants and defects of smaller sizes, which were once negligible, have become problematic.
For example, if a semiconductive device has dimensions of approximately 300 nm, a contaminant or defect having a dimension of 60 nm is typically negligible. However, as the feature size is reduced to 180 nm, this same defect may cause a defect in the manufactured device.
As a result of the problems described above, more stringent requirements have been placed on inspection devices used to detect defects in silicon wafers. The Semiconductor Equipment Manufacturing Industry consortium (SEMI) is an organization that defines defect sensitivity requirements for silicon wafer inspection devices. The sensitivity is typically defined in terms of the scanner""s ability to detect a polystyrene latex sphere (PSL) of a selected diameter. As an example of the increased sensitivity requirements, SEMI has recently released guidelines requiring that an inspection device maintain reliable detection capability of a 65 nm PSL defect for a 130 nm device geometry.
These increased sensitivity requirements have been problematic for many conventional silicon wafer inspection devices. Specifically, many conventional silicon wafer inspection devices use laser-based scanners and photomultiplier-based optical detectors to inspect the surface of silicon wafers for defects. While optical inspection systems have been capable of detecting 100 nm defects on silicon wafers, their performance is limited by quantum-mechanical shot noise. This noise is caused by the statistical probability associated with detecting the photons received from the scattered light that is reflected from the wafer surface. This susceptibility to noise limits the inspection system""s sensitivity to small particles that can cause device defects. In particular, the signal produced by particles decreases non-linearly with decreasing particle size.
At least one problem with the introduction of shot noise in silicon wafer inspection devices is illustrated in FIG. 1. FIG. 1 illustrates a portion of a conventional silicon wafer inspection system. This conventional silicon wafer inspection system 10 includes a light source 12 such as laser for directing an optical beam B toward the surface S of a workpiece W. The silicon wafer inspection system also includes a dark channel detector system 18 that may include several individual detectors for detecting scattered light reflected from the surface of the workpiece. The conventional silicon wafer inspection system also includes a light channel detector 20 positioned at an angle xcex2 that corresponds to, i.e., typically equal to, the angle xcex2 the beam makes with respect to the workpiece. The light channel detector detects specularly reflected light from the surface of the workpiece.
Importantly, the silicon wafer inspection device also includes a deflector 14 and an optical lens 16. The deflector and lens are positioned to receive the optical beam and deflect the beam so as to form a scan region 22 on the surface the workpiece. The scan region is defined by an in-scan dimension 24 that relates to the scan path of the light beam. Specifically, the scan path is the sweep of the optical beam through an angle cc controlled by the deflector. Further, the scan region includes a cross-scan dimension 26 that is associated with the width of the beam.
In operation, the workpiece or the laser is usually moved in relation to the optical beam such that the laser scans either all or a substantial portion of the surface of the workpiece. As the workpiece is scanned, the dark and light channel detectors receive optic signals reflected from the surface of the workpiece. These optic signals are provided to a signal processing device 28 for determining whether the workpiece includes defects.
As stated previously, a problem associated with many of these conventional systems is the introduction of shot noise into the dark channels of the silicon-wafer inspection system. This noise can overshadow small defects in the workpiece, thereby decreasing the sensitivity of the silicon-wafer inspection device. For example, FIG. 2 is a plot of the data signal received by a photomultiplier dark channel detector from a scan of a section of a workpiece by a conventional inspection device. This section of the workpiece includes a 100 nm PSL defect that is illustrated in the center of the scan by peak 30. Importantly, as illustrated by the plot, the optical signal received by the dark channel detector includes a considerable amount of noise that may overshadow smaller defects in the workpiece. Specifically, the amplitude of the signal generated by a defect is typically related to the diameter D of the defect by following equation:
xe2x80x83Amplitude of Defectxe2x88x9dDiameter6
As such, as the diameter of the defect decreases, the amplitude of the signal created by the defect decreases dramatically, thereby making the detection of smaller defects in a workpiece more problematic in a noisy inspection device.
In light of the problems associated with the introduction of signal noise in the dark and light channel detectors, silicon-wafer inspection systems have been developed to filter at least some of the noise from the received signals. These conventional inspection devices use signal filtering techniques to reduce the amount of noise in the optical signal received from the silicon wafer. These filtering techniques advantageously increase the sensitivity of the inspection device. However, these conventional inspection devices may not provide adequate filtering of the signal to reliably detect defects in the range of 65 nm as required by the SEMI guidelines.
Specifically, these conventional inspection devices filter the optical signal in the in-scan direction 24 of the scan region. The in-scan filter located in the signal processing device 28 is typically a conventional analog filter, and as further discussed below, is typically designed to match the light intensity characteristics of the optical beam B, such as a typical Gaussian light intensity distribution. As shown in FIG. 3, the in-scan filter filters the optical signals received by the dark and light channel detectors and effectively reduces the introduction of signal noise. Specifically, as shown in FIG. 3, which illustrates the optical signal received by the dark channel detector after the filter, the signal contains less noise. Further, the peak 30 indicating the defect is much more pronounced relative to the background shot noise.
As illustrated, the wafer inspection device effectively reduces some of the signal noise received by the light and dark channel detectors, thereby making the wafer inspection device more sensitive. However, as illustrated in FIG. 3, the filtered optical signal still includes considerable optical shot noise that may overshadow the optical signals produced by smaller dimensioned defects. As such, it would be desirable to provide an inspection system that has reduced noise to thereby increase the sensitivity of the inspection device.
A method and apparatus are therefore provided that scan a workpiece for defects with increased resolution and sensitivity relative to conventional techniques. In this regard, the method and apparatus of the present invention repeatedly scan different portions of the workpiece and the reflected signals are filtered in a cross-scan direction, that is, in a direction generally perpendicular to the direction in which the light beam is scanned. As such, the method and apparatus of the present invention effectively remove a significant portion of the noise in the reflected signals by filtering in the cross-scan direction. By appropriately filtering the reflected signals, the method and apparatus of the present invention can also take into account differences in the magnitude of the light that illuminates different portions of the workpiece in the cross-scan direction by filtering the reflected signals in the cross-scan direction in a manner that matches the light intensity distribution. As such, the method and apparatus of the present invention can detect defects in or on the workpiece with greater precision and greater resolution due to the filtering of the reflected signals in the cross-scan direction. Thus, the method and apparatus of the present invention can reliably detect smaller particles or other smaller defects on the surface of a workpiece than conventional defect detection techniques.
According to one advantageous embodiment, the workpiece is initially illuminated by a light source that provides an optical beam having in-scan and cross-scan dimensions defining a scan zone. In particular, each of a plurality of different portions of the workpiece is sequentially illuminated in a predetermined scan direction to thereby define a plurality of scans. As a point of reference, the in-scan dimension of the optical beam extends parallel to the predetermined scan direction and the cross-scan dimension of the optical beam is perpendicular to the predetermined scan direction. At least some of the optical signals that illuminate the workpiece are reflected during each of the scans. The reflected signals are typically received and collected by optical receivers or detectors. A digital data set is then constructed for each scan of the workpiece. The data set for each scan is then filtered, typically by the digital cross-scan filter, in the cross-scan dimension of the optical beam based upon at least one data set corresponding to the optical signals received during another scan. In other words, the data set for one scan is filtered in the cross-scan dimension based upon another data set corresponding to another scan.
In one advantageous embodiment, a data set is constructed for each respective scan that has individual data points representing the optical beam reflected from the workpiece at different respective positions along the scan direction. In order to filter the data set, each data point of the scan is preferably individually filtered based upon a corresponding data point of a data set representing the optical signals received during another scan to thereby filter the data set in the cross-scan dimension of the optical beam.
In a further advantageous embodiment, each of the individual data points of the data set are filtered based on a plurality of other data sets representing the optical signal received during a plurality of respective scans occurring prior to and after the present scan. In this embodiment, the individual data points of the data set of the present scan are individually added to corresponding individual data points of the plurality of data sets corresponding to scans of the workpiece occurring prior to and after the present scan to thereby filter the data set of the respective scan in the cross-scan dimension of the optical beam.
Typically, the workpiece is illuminated with an optical beam having a predetermined light intensity distribution in the cross-scan dimension such that different positions of the workpiece in a respective scan are illuminated with light having different magnitudes in the in-scan and cross-scan dimensions. In order to filter the data set in the cross-scan dimension, the filter must be matched to the light intensity distribution of the optical beam. To match the filter to the optical beam, an adjusted data set is therefore generated for each scan that accounts for the predetermined light intensity distribution of the optical beam in the cross-scan dimension of the optical beam. In particular, the adjusted data set can be generated, typically by the cross-scan filter, by multiplying the individual data points of the data set by a predefined cross-scan coefficient that accounts for the differences in the magnitude of light that illuminates the different positions of the workpiece in the respective scan in the cross-scan dimension.
In addition to multiplying the individual data points of a data set by a predefined cross-scan coefficient such that the filter matches the light density distribution of the optical beam, each data point of the adjusted data set of the respective scan is also filtered in the cross-scan direction. The data points of the adjusted data set are filtered in the cross-scan direction by individually adding each data point of the adjusted data set to corresponding data points of another adjusted data set corresponding to an optical signal received by the receiver during another scan, where the adjusted data set has been adjusted by another predefined cross-scan coefficient. By adding the individual data points of the adjusted data set of the present scan that has been adjusted by a cross-scan coefficient to the individual data points of another adjusted data set of another scan that has been adjusted by another cross-scan coefficient, a filtered data set is created that has been filtered in the cross-scan dimension of the optical beam. In order to further filter the data set in the cross-scan dimension, each data point of the adjusted data set of the present scan is individually added to corresponding data points of a plurality of adjusted data sets that have each been adjusted by predefined cross-scan coefficients.
In one embodiment, the workpiece is illuminated with an optical beam having a predetermined Gaussian light intensity distribution. As such, portions of the workpiece located in a middle portion of the scan are illuminated with light having a greater intensity than portions of the workpiece located on opposite end portions of the scan in the cross-scan dimension. In order to match the cross-scan filter to the light intensity distribution of the optical beam, the individual data points of a data set are multiplied by a predetermined cross-scan coefficient that accounts for the differences in the magnitude of light that illuminates the different positions of the workpiece in the respective scan in the cross-scan direction.
In addition to adjusting the data set of the scan so that the filter is matched to the light intensity distribution of the optical beam, in one embodiment, the data set is also filtered in the cross-scan dimension of the optical beam. In this embodiment, the data set is filtered based on a plurality of data sets corresponding to scans of the workpiece occurring prior to and after the present scan. To match the cross-scan filter, for each of the plurality of scans, an adjusted data set is generated to account for the predetermined Gaussian light intensity distribution of the optical beam in the cross-scan dimension. Each of the adjusted data sets is generated by multiplying the individual data points of each data set by a respective predefined cross-scan coefficient. Data sets representing scans closer in time to the present scan are multiplied by greater scan-coefficient values than the individual data points representing data sets corresponding to scans occurring further in time from the present scan. After each of the plurality of data sets have been adjusted, the present scan is filtered in the cross-scan dimension of the optical beam by adding the corresponding individual data points of the plurality of adjusted data sets corresponding to scans of the workpiece occurring prior to and after the respective scan to the individual data points of the adjusted data set of the present scan.
In addition to filtering the reflected optical signals in a cross-scan direction, the optical signals received during a respective scan can also be filtered in the in-scan dimension of the optical beam, typically by means of an in-scan filter. Like the cross-scan filtering, the in-scan filter is matched to filter the optical signals based on the predetermined light intensity distribution of the optical beam in the in-scan dimension. For example, in instances in which the workpiece is illuminated with an optical beam having a predetermined Gaussian light intensity distribution, the portions of the workpiece located in a middle portion of a scan are illuminated with light having greater intensity than portions of the workpiece located on the opposed end portions of the scan in the in-scan dimension. As such, the in-scan filter is matched to the light intensity distribution of the optical beam, such that the in-scan filter filters the optical signals accordingly.
The method and apparatus of one embodiment of the present invention can also remove optical noise in the scan caused by irregularities in the surface of the workpiece. In this regard, a data set for each respective scan is received, typically by a processor. As described above, the data set has individual data points representing the optical beam reflected from the workpiece at different positions along the scan direction. According to this embodiment, a predetermined number of data points are selected from each of a plurality of respective scans. An average scan is then generated, typically by the processor. The average scan has individual data points representing the average optical beam reflected from the workpiece at different positions along the scan direction for the plurality of scans. Individual data points of the average scan are then subtracted, typically by the processor, from the respective individual data points of each data set for each of the plurality of scans. As such, each data set can be corrected for irregularities in the surface of the workpiece.
The method and apparatus of another embodiment of the present invention can determine whether the workpiece contains a defect. In this embodiment, each data point of the respective scan is compared to a threshold value, typically by means of a processor. Thereafter, portions of the workpiece that have corresponding data points in the scan that are at least as great as the threshold value are identified as potential defects.
The method and apparatus of another embodiment of the present invention can also compensate for consistent variations in the intensity of the optical signal. In this regard, a repetitive or systematic signature data set can be subtracted from the data set that represents the optical signals that reflected from the workpiece at different positions along the scan direction. Typically, the signature data set includes individual data points representing the consistent variations in the intensity of the optical signal. Thus, the resulting data set can more accurately represent any defects in or on the surface of the workpiece.
According to the present invention, the method and apparatus filters the reflected optical signals in the cross-scan dimension and, accordingly, removes noise from the reflected optical signals. In addition, the method and apparatus of the present invention effectively compensates for the predetermined light intensity distribution of the optical beam, such as a predetermined Gaussian light intensity distribution, when filtering the optical signal in the cross-scan dimension. By filtering the reflected optical signals in the cross-scan dimension, the resulting filtered data set more accurately represents defects in or on the surface of the workpiece such that smaller defects can be identified with greater precision than conventional defect detection techniques.